1. Field of the Invention
The present invention relates to an optical fiber represented by a silica glass based optical fiber and an optical fiber manufacturing method and in particular, to a technique of reducing the polarization mode dispersion (hereinafter, referred to as “PMD”) of an optical fiber. Especially, the invention relates to an optical fiber, in which the amount of increase in PMD is small even if subject to interference, such as lateral pressure or bending, and a method and apparatus for manufacturing optical fiber.
Priority is claimed on Japanese Patent Application No. 2011-028387, filed Feb. 14, 2011, the content of which is incorporated herein by reference.
2. Description of Related Art
As is well known, a PMD is a phenomenon in which a propagation time difference (delay difference) occurs between two orthogonal polarization mode components in an optical fiber.
In addition, if a PMD increases, waveform deterioration occurs in signal light transmitted through the fiber in digital transmission, therefore, it becomes difficult to separate adjacent pulses from each other. As a result, a problem occurs in that problems arise such as the transmission capacity becoming limited.
Therefore, suppressing a PMD as much as possible is desired.
In addition, a PMD is caused by the optical anisotropy of an optical fiber. The causes of the occurrence of PMD are largely divided into internal factors, in which the optical anisotropy is caused by the internal structure, material, and the like of the optical fiber, and external factors, in which the optical anisotropy is caused by the stress from the outside of the optical fiber and the like.
The most high-impacting of the internal factors is the cross-sectional shape of the optical fiber.
On the other hand, in the manufacture of optical fibers, it is difficult in practice to realize a completely circular cross-sectional shape including the core of the optical fiber and the cladding around the core regardless of which fiber preform manufacturing method and method of manufacturing a bare optical fiber by drawing (fiber drawing) a fiber preform are selected.
Therefore, the actual product has a cross-sectional shape distorted to, for example, a slightly elliptical shape.
If the anisotropy of such a cross-sectional shape becomes large, the refractive index distribution in the cross section is no longer a completely concentric circle. Accordingly, birefringence occurs, and this increases PMD.
On the other hand, stress applied anisotropically, such as stress caused by bending or lateral pressure applied to the optical fiber from the outside, may be mentioned as the most high-impact of the external factors. The birefringence also occurs due to such anisotropic stress applied from the outside, and this increases PMD.
By the way, in order to reduce PMD of the optical fiber, applying torsion to the optical fiber is effective, and methods disclosed in Japanese Unexamined Patent Application Publication No. H8-295528, U.S. Pat. No. 6,324,872, WO2009/107667, Japanese Unexamined Patent Application Publication No. 2010-122666, and U.S. Pat. No. 7,317,855 have been proposed.
Among them, Japanese Unexamined Patent Application Publication No. H8-295528 and U.S. Pat. No. 6,324,872 disclose a method of applying torsion before an optical fiber preform is solidified, at the time of drawing of a bare optical fiber, so that the torsion is permanently fixed.
The above method is a method of giving a bare optical fiber the torsion as plastic deformation (plastic torsion) so that the torsion is maintained as it is, even if the external force on the optical fiber is removed, that is, a method of maintaining the torsional state as permanent deformation.
Hereinafter, such a plastic torsion which remains as permanent deformation may be called a “span”.
On the other hand, WO2009/107667, Japanese Unexamined Patent Application Publication No. 2010-122666, and U.S. Pat. No. 7,317,855 disclose a method of applying the torsion to an optical fiber after the optical fiber is drawn and solidified.
The torsion in this case occurs due to elastic deformation.
That is, torsion in this case is elastic torsion which returns to the state before twisting when the external force is removed and accordingly the optical fiber returns to a free state (external force removal state).
In this case, using the optical fiber in an end product, such as a cable, finally in a state where the elastic torsion is held, that is, using the optical fiber in a state where the elastic torsion is held as an optical fiber used in the end product, such as a cable, is assumed.
Hereinafter, such elastic torsion may be called a “twist”. As described above, the causes of the occurrence of PMD are largely divided into internal factors and external factors, and the method of giving an optical fiber a span (plastic torsion) which is disclosed in Japanese Unexamined Patent Application Publication No. H8-295528 and U.S. Pat. No. 6,324,872 is effective for PMD caused by the internal factors.
However, it is known that such a method of giving an optical fiber a span is not effective for suppressing a PMD increase caused by external factors (for example, refer to WO2009/107667).
On the other hand, the method of giving a twist (elastic torsion) as disclosed in WO2009/107667, Japanese Unexamined Patent Application Publication No. 2010-122666, and U.S. Pat. No. 7,317,855 is effective for suppressing PMD increase caused by external factors, such as lateral pressure or bending.
However, the above twist returns to the state before twisting due to elastic force when the external force is removed.
Here, the external force, such as frictional force, applied to an optical fiber may be removed or the external force, such as frictional force, may become significantly small, for example, in a coloring process, a process of arraying a plurality of optical fibers in a tape form, and an actual mass production process including the process of forming optical fiber cables and the inter-processes, which are all processes for making twisted optical fibers end products such as optical cables.
In above case, since the torsion is removed or the torsion becomes significantly small, the effect of suppressing a PMD increase caused by external factors disappears.
Therefore, there has been a problem in that it is difficult to reliably and stably suppress a PMD increase caused by external factors in end products such as cables.
As described above, in the related art, it has been difficult to reliably and stably suppress a PMD increase, which is caused by external factors such as anisotropic external forces as exemplified by lateral pressure or bending stress applied to optical fibers, in end products.